MapReduce 学习5 ---- 输出文件学习

2.reduce任务处理

2.3 把reduce的输出保存到文件中。

ruduce任务中把数据写到hdfs中使用

context.write(k2, new LongWritable(sum));

只要分析context，就可以理解如何把数据写到hdfs上

class ChainReduceContextImpl<KEYIN, VALUEIN, KEYOUT, VALUEOUT> implements
    ReduceContext<KEYIN, VALUEIN, KEYOUT, VALUEOUT> {


    private final RecordWriter<KEYOUT, VALUEOUT> rw;


    public void write(KEYOUT key, VALUEOUT value) throws IOException,
      InterruptedException {
       rw.write(key, value);
    }


}


//分隔符的配置项
public static String SEPERATOR = "mapreduce.output.textoutputformat.separator";
protected static class LineRecordWriter<K, V>
    extends RecordWriter<K, V> {

   private static final byte[] newline;
    static {
      try {
        //设置每一行的换行符
        newline = "\n".getBytes(utf8);
      } catch (UnsupportedEncodingException uee) {
        throw new IllegalArgumentException("can't find " + utf8 + " encoding");
      }
    }

    protected DataOutputStream out;
    private final byte[] keyValueSeparator;

    public LineRecordWriter(DataOutputStream out, String keyValueSeparator) {
      this.out = out;
      try {
        this.keyValueSeparator = keyValueSeparator.getBytes(utf8);
      } catch (UnsupportedEncodingException uee) {
        throw new IllegalArgumentException("can't find " + utf8 + " encoding");
      }
    }

    public LineRecordWriter(DataOutputStream out) {
      //默认分隔符是空格
      this(out, "\t");
    }


     //把每一个k3,v3写到一行中，k3与v3默认以空格为分隔符
    public synchronized void write(K key, V value)
      throws IOException {

      boolean nullKey = key == null || key instanceof NullWritable;
      boolean nullValue = value == null || value instanceof NullWritable;
      if (nullKey && nullValue) {
        return;
      }
      if (!nullKey) {
        writeObject(key);
      }
      if (!(nullKey || nullValue)) {
        out.write(keyValueSeparator);
      }
      if (!nullValue) {
        writeObject(value);
      }
      out.write(newline);
    } 

    private void writeObject(Object o) throws IOException {
      if (o instanceof Text) {
        Text to = (Text) o;
        out.write(to.getBytes(), 0, to.getLength());
      } else {
        out.write(o.toString().getBytes(utf8));
      }
    }   

}